1. Field of the Invention
The present invention relates to acoustic transducer apparatus and methods for measuring a rate of flow of a fluid through a pipe. More specifically, the invention relates to arrangements in which ultrasonic transducers are clamped to an external surface of the pipe.
2. Background Information
Transit time flow meters used to measure the rate of flow of fluids through a pipe commonly employ clamp-on transducers attached against an external surface of the pipe. In these applications the transducers are commonly held against the pipe with screw-tensioned clamp bands. When these bands are tightened during installation, unbalanced frictional forces can move the transducer tangential to the pipe and can torque a portion of the transducer away from intimate acoustic contact with the pipe. Moreover, clamp band tension can change due to temperature or other factors, thus leading to flow sensor drift and loss of precision and reliability.
Current art clamp-on transducers typically reflect a compromise between acoustic coupling and mechanical mounting considerations. Some degradation of coupling is accepted in exchange for mechanical convenience. For example, a relatively large mounting area between the transducer and the pipe may be provided in order to bring an acceptable degree of mechanical stability to the transducer mounting. This also means that less of the available clamping force is applied directly to the area transmitting the acoustic energy. Hence, acoustic coupling promoters, such as silicone greases, can play a bigger role in maintaining the acoustic energy path. Because coupling promoters tend to evaporate, wick away or otherwise degrade or leave the area over a period of time, the transducer mounting may require frequent maintenance effort.
In some known arrangements, clamp-on transducers are made with a reduced height so as to press more evenly against the pipe. In these cases, the increase in clamp band tension needed for a reliable acoustic connection tends to become excessive, thus dissipating any advantage gained by this procedure.
It is further noted that uneven movement in a clamp band as it is tightened can move a transducer along the pipe. When this happens, the clamp band must be loosened and the clamping process restarted.